In large endless non-metallic conveyor belts used to convey bulk material, there is a possibility of encountering a rip in the belt, for example, by a sharp object dropped thereon at the loading station. It is desirable promptly to detect such rips and, preferably, to shut down the conveyor belt upon such detection, thereby minimizing damage to the belt. One such conveyor belt rip detector is disclosed in U.S. Pat. No. 3,792,459. In such rip detector plural antennas, which may be single electrical conductors, are embedded in the belt transversely to its length at spaced-apart locations in the belt. An electrical signal is coupled by respective antennas from a transmitter to a receiver as the belt moves and the respective antennas pass in capacitive coupling relation with the transmitter and receiver at a rip detector station, and the receiver thus delivers an input signal to detector circuitry which interprets the same as an indication of satisfactory belt integrity. However, a broken antenna, for example at a place where the belt has been ripped, will not couple the transmitter signal through to the receiver, and the detector then senses the same as an indication of the occurrence of a rip condition. The detector circuit in such patent is operative after the lapse of a predetermined time period corresponding to the passage of a given number of broken antennas past the rip detector station to produce a distinguishable output that activates an alarm and/or deactivates the conveyor belt drive.
A number of improvements for use in conveyor belt rip detectors are disclosed in co-pending U.S. patent application Ser. No. 960,424, filed Nov. 13, 1978, for "Monitor System and Method for Detecting Sequential Events in a Cyclical Process", now U.S. Pat. No. 4,228,513, issued Oct. 14, 1980, and assigned to the same assignee as the present application. One of such improvements includes a means for detecting the progress of the conveyor belt to know when an antenna should be at the rip detector station. If there is no antenna present then, a prompt shut-down of the conveyor belt drive may be effected. Improvements in the monitor system and method disclosed in U.S. patent application Ser. No. 960,424 now U.S. Pat. No. 4,228,513, issued Oct. 14, 1980, are disclosed in copending commonly assigned U.S. patent application Ser. No. 126,218, filed Mar. 3, 1980, of Frank A. Doljack entitled "Signal Detector Circuit With Gain Control Gain now U.S. Pat. No. 4,349,883.
The disclosures of such copending U.S. patent applications Ser. No. 960,424 now U.S. Pat. No. 4,228,513 issued Oct. 14, 1980 and Ser. No. 126,218 now U.S. Pat. No. 4,349,883 are hereby incorporated by reference.
It has been found that due to wear of antennas in a conveyor belt ultimately possibly causing a break in the antenna, the signal coupling efficiency of the antenna may decrease to a marginal level or signal coupling may even terminate completely, even though the actual integrity of the conveyor belt itself is undisturbed. In the past such antenna problems would cause a nuisance shut-down (stopping) of the conveyor belt requiring a deliberate re-start of the belt by a field worker even though nothing catastrophic had occurred to the belt. Such nuisance shut-downs are costly in terms of lost productivity and wasted labor to re-start the system.
One prior approach to reduce nuisance shut-downs was to require the rip detector to sense that there are five or even more sequential antennas missing in the belt before stopping the belt. However, a drawback to this approach is the loss in sensitivity or accuracy of the rip detector equipment as it monitors the entire belt and, in the case of a real rip, an increase in the length thereof before shut-down occurs.
Thus, it would be desirable to avoid nuisance shut-downs of a monitored cyclical process while minimizing loss of sensitivity or accuracy of the monitoring function.